1. Field of the Invention
The present invention relates to an oil pump in which the number of teeth is increased to inhibit noise generation and durability is also improved.
2. Description of the Related Art
A problem associated with a rotor with a small number of teeth is that the discharge amount per one cell increases, thereby increasing pulsations, causing vibrations of the oil pump body and the like, and generating noise. A method of increasing the number of teeth is often used to reduce pulsations and inhibit noise. Japanese Patent Application Publication No. 2007-85256 describes the configuration in which the number of teeth is increased by comparison with that of the teeth with a typical toroidal profile.
In the configuration described in Japanese Patent Application Publication No. 2007-85256, the number of teeth is increased by reducing the so-called tooth size, which is a size from the tooth tip to the tooth bottom. Thus, the tooth profile described in Japanese Patent Application Publication No. 2007-85256 is squeezed radially with respect to the typical toroidal tooth profile. Because of such a profile, the region close to the dot-dash line (base circle A) shown in FIG. 1 of Japanese Patent Application Publication No. 2007-85256, that is, the intermediate region between the tooth tip and tooth bottom, protrudes circumferentially outward relative to other regions.
Since the inner rotor 10 and the outer rotor 20 should rotate without minimum limit cutting into each other, the respective outer teeth and inner teeth thereof have profiles that are hollowed out more than the usual tooth profiles. The outer teeth 11 of the inner rotor 10 disposed in the left-right direction in FIG. 1 of Japanese Patent Application Publication No. 2007-85256 and the inner teeth 21 (two teeth on the right side and one on the left side) of the outer rotor 20 are in contact, but other teeth, that is, the teeth disposed in two locations on the upper side and two locations on the lower side in FIG. 1, are not in contact and large gaps are opened therebetween.
The problem associated with the oil pump rotor of Japanese Patent Application Publication No. 2007-85256 having such a configuration is that since the number of outer teeth 11 of the inner rotor 10 and the number of inner teeth 21 the outer rotor 20 that are in contact with each other is small (three), larger stresses (forces) are generated in the contact portions and the durability of the rotor decreases.
In an oil pump with a tooth profile having a toroidal shape that has been widely used, the outer teeth of the inner rotor and the inner teeth of the outer rotor are all in contact with each other.
In other words, the problem associated with the decrease in rotor durability is not encountered when the tooth profile has a toroidal shape. Such a problem arises because not all of the teeth are in contact when a tooth profile of a non-toroidal shape is used as a measure to increase the discharge amount or efficiency.